Molecular Layer Deposition Approach for Hybrid Metalorganic-Inorganic Systems— Thin Film Oxides Functionalized with a Luminescent Europium Complex Monolayer

The organic-inorganic hybrid systems have attracted great attention because of their potential applications in several fields of material science such as sensors, dye sensitized solar cells (DSSCs), optoelectronic devices, and heterogeneous catalysis. In these systems, transition metal oxides thin films are the most promising inorganic materials, due to their wide spectrum of magnetic, electrical, and optical properties. Nickel oxide has been chosen as the inorganic component of the hybrid system due to its multifunctional properties such as high dielectric constant, low resistivity and UV optical transparency.<br/>The most common functional organic moieties chosen for the hybrid approach are the molecules having silane, thiol and carboxylic functionalities as anchoring groups. Among these, the attention has been devoted to luminescent lanthanide complexes due to the exceptional photophysical properties such as high quantum yields and sharp emission profiles related to f–f electronic transitions. Eu(III) complexes have attracted growing interest as efficient downshifter and as a probe to sense the chemical environment.<br/>For a variety of applications, the possibility of a fast, high efficiency and reproducible assembly method of molecules into nanostructured solid substrates is of great importance. Molecular layer deposition (MLD) is a solvent free method to create conformal coatings of organic molecules layers through self-limiting reactions on the surface and with an excellent molecular-level control of thickness and composition.<br/>In the present work, we applied for the first time a full vapor phase approach based on the sequential steps of (i) Metal-Organic Chemical Vapor Deposition (MOCVD) of the inorganic NiO thin films, and (ii) MLD to link, in a covalent way, on the activated surface an Eu(β-diket)₃L complex, where β-diket is a β-diketone and L is a Lewis base. An accurate X-ray photoelectron characterization confirmed the optimal parameter conditions of the activation step and of the covalent anchoring of the luminescent europium(III) adduct on the NiO films. Emission spectroscopy of the hybrid NiO/Eu(III) system shows that an accurate control of the Eu(III) coordination sphere may tune the luminescence of the monolayers.